The present invention relates to an ink-jet head for jetting ink by using a piezoelectric effect of a piezoelectric device and a method of manufacturing the same.
Recently, ink-jet printers are widely used in offices and households. Various systems have been proposed for ink-jet heads used in the ink-jet printers in order to meet recent demands for low noise and high print quality. In general, the systems for the ink-jet heads can be roughly divided into the following two systems:
In a first system, part of an ink passage and an ink chamber is formed into a pressure chamber by using a piezoelectric actuator having a piezoelectric device, and a pulse voltage is applied to the piezoelectric device so as to deform the piezoelectric actuator. Thus, the pressure chamber is deformed to have a smaller volume, thereby generating a pressure pulse within the pressure chamber. By using the pressure pulse, ink drops are jetted through a nozzle hole communicating with the pressure chamber.
In a second system, an exothermic resistance is provided in an ink passage, and a pulse voltage is applied to the exothermic resistance so as to generate heat therein. Thus, ink contained in the passage is boiled with vapor bubble generated. By using the pressure of the vapor bubble, ink drops are jetted through a nozzle hole.
The present invention relates to an ink-jet head of the first system, and hence, this system will be further described in detail. FIGS. 7 through 9 show an exemplified conventional ink-jet head of the first system, and the ink-jet head comprises a head body 101 including a plurality of pressure chamber concaves 102 each having a supply port 102a for supplying ink and a discharge port 102b for discharging ink. The concaves 102 of the head body 101 are arranged along one direction at predetermined intervals.
The head body 101 includes a pressure chamber part 105 forming the side walls of the concaves 102, an ink passage part 106 forming the bottoms of the concaves 102 and including plurality of thin plates adhered to one another, and a nozzle plate 113. Within the ink passage part 106, an ink supply passage 107 communicating with the supply port 102a of each concave 102 and an ink discharge passage 108 communicating with the discharge port 102b of each concave 102 are formed. Each ink supply passage 107 communicates with an ink supply chamber 110 extending in the direction of arranging the concaves 102, and the ink supply chamber 110 communicates with an ink supply hole 111 formed in the pressure chamber part 105 and the ink passage part 106 and connected with an external ink tank (not shown). In the nozzle plate 113, nozzle holes 114 respectively connected with the ink discharge passages 108 are formed.
On the upper surface of the pressure chamber part 105 of the head body 101, a piezoelectric actuator 121 is disposed. The piezoelectric actuator 121 includes one common electrode 122 of Cr that covers all the concaves 102 of the head body 101 so as to form pressure chambers 103 together with the concaves 102 and is shared by all piezoelectric devices 123 described below. The common electrode 122 also works as the so-called vibration plate. Furthermore, the piezoelectric actuator 121 includes the piezoelectric devices 123 of lead zirconate titanate (PZT) disposed on the upper surface of the common electrode 122 correspondingly to the respective pressure chambers 103, and an individual electrode 124 of Pt disposed on each piezoelectric device 123 for applying a voltage to the corresponding piezoelectric device 123 together with the common electrode 122.
When a pulse voltage is applied between the common electrode 122 and each individual electrode 124, each piezoelectric device 123 shrinks in a lateral direction perpendicular to a thickness direction, but the common electrode 122 and the individual electrode 124 do not shrink. Therefore, a portion of the common electrode 122 corresponding to the piezoelectric device 123 is deformed into a convex projecting toward the pressure chamber 103 due to the so-called bimetal effect. This deformation causes a pressure within the pressure chamber 103, and owing to the pressure, ink contained in the pressure chamber 103 is jetted from the nozzle hole 114 through the discharge port 102b and the ink discharge passage 108.
In the ink-jet head for jetting ink by using the piezoelectric actuator 121 as described above, various improvements have been recently made so as to meet strict demands for compactness and light weight, a low driving voltage, low noise, low cost, and high controllability in jetting ink. In order to attain further compactness and higher performance, the common electrode 122, the piezoelectric devices 123 and the individual electrodes 124 can be formed from thin films easily subjected to refined processes.
In this case, for example, a method of manufacturing an ink-jet head shown in FIGS. 10(a) through 10(g) can be adopted. In FIGS. 10(a) through 10(g), the ink-jet head is shown upside down, namely, inversely to that shown in FIGS. 7 and 8.
Specifically, a Pt film 142 is formed on the entire surface of a supporting substrate 141 of MgO as is shown in FIG. 10(a), and then, the Pt film 142 is patterned (separated), thereby forming a plurality of individual electrodes 124 as is shown in FIG. 10(b).
Subsequently, a PZT film 143 is formed on the entire supporting substrate 141 bearing the individual electrodes 124 as is shown in FIG. 10(c), and the PZT film 143 is patterned into the same shape as the Pt film 142. Thus, a plurality of piezoelectric devices 123 are formed as is shown in FIG. 10(d).
Next, on the piezoelectric devices 123, a common electrode 122 (of a Cr film) is formed as is shown in FIG. 10(e), and the common electrode 122 is fixed on a pressure chamber part 105 as is shown in FIG. 10(f).
Then, the supporting substrate 141 is melted and removed by using heated phosphoric acid or the like, and the pressure chamber part 105 is fixed on an ink passage part 106 and a nozzle plate 113 previously integrated as is shown in FIG. 10(g). Thereafter, wiring of the individual electrodes 124 and other necessary processes are conducted, resulting in completing the ink-jet head.
In the above-described method of manufacturing an ink-jet head, however, it is particularly difficult to form the common electrode 122 in the shape of a thin film. Specifically, in a method where the common electrode 122 is formed by adhering a previously formed Cr film onto the piezoelectric devices 123 with an adhesive, the film is so thin that it is difficult to adhere it onto the piezoelectric devices 123. On the other hand, in a method in which the common electrode 122 is directly formed on the piezoelectric devices 123 by sputtering or the like, good adhesion can be attained and the thickness can be very small. However, the common electrode 122 cannot be formed into a flat shape on the entire surface of the supporting substrate 141 because the portion thereof on the supporting substrate 141 where the individual electrodes 124 and the piezoelectric devices 123 are not formed is placed at a lower level. Specifically, a portion of the common electrode 122 corresponding to an interval between the piezoelectric devices 123 can be formed in a lower level down to the surface of the supporting substrate 141 as is shown in FIG. 11(a). As a result, portions of the common electrode 122 corresponding to the respective piezoelectric devices 123 can be separated from one another. Alternatively, the portion of the common electrode 122 corresponding to the interval between the piezoelectric devices 123 can be largely bent toward the supporting substrate 141 as is shown in FIG. 11(b). When the common electrode 122 is separated as described above, it is troublesome because the separated portions of the common electrode 122 should be electrically connected through a wire. When the common electrode 122 is bent as described above, the displacement characteristic is varied in the piezoelectric actuator 121 and the common electrode 122 can be easily damaged.
In the above-described manufacturing method, another method can be adopted as follows instead of patterning the PZT film 143: The common electrode 122 is formed on the entire PZT film 143, and the common electrode 122 is fixed on the pressure chamber part 105. After removing the supporting substrate 141, the PZT film 143 is patterned on its face on the same side as the individual electrodes 124, thereby forming the piezoelectric devices 123. This method is not impossible but is actually difficult to adopt because the common electrode 122 and the piezoelectric devices 123 cannot resist heat applied during the patterning. In particular, when the common electrode 122 and the piezoelectric devices 123 have small sizes, this method is further difficult to adopt.
The present invention was devised in view of the aforementioned problems and disadvantages, and an object of the invention is attaining compactness of an ink-jet head for jetting ink by using the piezoelectric effect of a piezoelectric device with the displacement characteristic and durability of a piezoelectric actuator improved as far as possible.
In order to achieve the object, according to the invention, a plane where a common electrode is to be formed is flattened before forming the common electrode.
Specifically, the method of manufacturing an ink-jet head for jetting ink by using a piezoelectric effect of a piezoelectric device of this invention comprises the steps of forming plural individual electrodes and plural piezoelectric devices stacked in this order on a supporting substrate; flattening a top surface of the supporting substrate including the individual electrodes and the piezoelectric devices by filling a filler in a portion on the supporting substrate where the individual electrodes and the piezoelectric devices are not formed up to substantially the same level as a level of upper surfaces of the piezoelectric devices; forming a common electrode on the entire flattened top surface of the supporting substrate; fixing a pressure chamber part for forming pressure chambers on the common electrode; and removing the supporting substrate after fixing the pressure chamber part on the common electrode.
In this manner, the top surface of the supporting substrate where the common electrode is to be formed is flattened, and hence, the common electrode can be formed uniformly in a flat shape on the entire top surface of the supporting substrate by sputtering or vacuum evaporation. As a result, even a compact ink-jet head can be free from variation of the displacement characteristic of the piezoelectric actuator and damage of the common electrode. Thus, the ink-jet head attains high ink-jetting performance and high durability.
In the method of manufacturing an ink-jet head, the filler is preferably made from an organic resin, a photosensitive resin or polyimide. Thus, the filler can be easily filled by spin coating or the like, resulting in improving the productivity.
Alternatively, the filler can be made from an inorganic insulating material. Thus, the filler of SiO2 or the like can be easily formed by sputtering or the like, and the filler can attain high environment resistance and reliability.
When the filler is made from an inorganic insulating material, the step of flattening the top surface of the supporting substrate preferably includes steps of forming an inorganic insulating material film on the entire top surface of the supporting substrate; and removing, by lapping followed by polishing, a portion of the inorganic insulating material film disposed above the upper surfaces of the piezoelectric devices. In this manner, the portion of the inorganic insulating material film disposed above the upper surfaces of the piezoelectric devices can be roughly abraded by lapping and mirror-ground by polishing. As a result, the entire top surface of the supporting substrate can be uniformly and definitely flattened.
In this case, the step of removing the portion of the inorganic insulating material film disposed above the upper surfaces of the piezoelectric devices preferably includes lapping by using abrasive grains of cerium oxide and polishing by using a non-metal soft material. Thus, even when there is a large difference in hardness between the inorganic insulating material to be abraded and the material for the piezoelectric devices, the entire top surface of the supporting substrate can be uniformly flattened.
Also, when the filler is made from an inorganic insulating material, the step of flattening the top surface of the supporting substrate can include steps of forming an inorganic insulating material film on the entire top surface of the supporting substrate; flattening an upper surface of the inorganic insulating material film by etch back; and removing a portion of the inorganic insulating material film, whose upper surface has been flattened, disposed above the upper surfaces of the piezoelectric devices. In this manner, the upper surface of the hard inorganic insulating material film can be easily flattened to some extent, resulting in easing removal of the inorganic insulating material film through lapping and polishing.
Furthermore, when the filler is made from an inorganic insulating material, the step of flattening the top surface of the supporting substrate can include steps of forming an inorganic insulating material film on the entire top surface of the supporting substrate by bias sputtering; and removing a portion of the inorganic insulating material film disposed above the upper surfaces of the piezoelectric devices. In this manner, the upper surface of the inorganic insulating material film can be flattened to some extent in forming the inorganic insulating material film by the bias sputtering, resulting in easing removal of the inorganic insulating material film through lapping and polishing.
Alternatively, the ink-jet head of this invention comprises a head body including plural concaves for pressure chambers each having a supply port for supplying ink and a discharge port for discharging ink; and a piezoelectric actuator including a common electrode covering the concaves for forming the pressure chambers together with the concaves; piezoelectric devices separately disposed on a surface of the common electrode opposite to the pressure chambers respectively correspondingly to the pressure chambers; individual electrodes separately disposed on surfaces of the piezoelectric devices opposite to the common electrode for applying a voltage to the piezoelectric devices together with the common electrode; and a filler filled in a portion on the surface of the common electrode opposite to the pressure chambers where the piezoelectric devices and the individual electrodes are not formed, for placing a surface of the filler opposite to the pressure chambers at substantially the same level as surfaces of the individual electrodes opposite to the pressure chambers, and the piezoelectric actuator is deformed, under application of a voltage to the piezoelectric devices through the individual electrodes and the common electrode, so as to reduce a volume of the pressure chambers, whereby allowing ink contained in the pressure chambers to be discharged through the discharge ports.
Owing to this structure, an ink-jet head having high ink-jetting performance and high durability can be manufactured by the aforementioned manufacturing method. Furthermore, the filler can protect the piezoelectric actuator from a mechanical external force derived from some accident or mis-operation as well as can make stress transmission between the common electrode and the side walls of the piezoelectric devices smooth. As a result, the life of the piezoelectric devices can be elongated.
In the ink-jet head, the filler is preferably made from an insulating material whose Young""s modulus is set to be 1/20 or less of a Young""s modulus of the piezoelectric devices. Thus, the filler can be substantially prevented from obstructing the operation of the piezoelectric actuator. As a result, the piezoelectric actuator can attain a very good displacement characteristic.
Furthermore, in the ink-jet head, the common electrode and the piezoelectric devices preferably have a thickness of 5 xcexcm or less. Thus, by adopting the aforementioned method, the effects of the invention of attaining a good displacement characteristic and high durability of a piezoelectric actuator can be maximumly exhibited in a compact ink-jet head.